The former head of Nelson Mandela Children's Fund, Jeremy Ractliffe, has said he did receive uncut diamonds from the model Naomi Campbell.
He issued a statement after her testimony at the war crimes trial of ex-Liberian leader Charles Taylor.
It is alleged she received the gems from Mr Taylor in 1997, which could link him to illegal "blood diamonds".
Mr Ractliffe said he was happy to testify in The Hague and had now handed the stones over to the authorities.
"Three small uncut diamonds were given to me by Naomi Campbell on the Blue Train on 26 September 1997," he said in a statement sent to the BBC.
South Africa's special Hawks police unit has since confirmed to the BBC that they received the stones from Mr Ractliffe on Thursday, who first made contact with them on Tuesday.
It said the diamonds were now being tested to find out their origin.
At the trial, Ms Campbell said she was given some "dirty-looking stones" after a 1997 charity dinner hosted by South Africa's former President Nelson Mandela where Mr Taylor was also a guest.
She said two unidentified men appeared at her room and gave her the stones.
She told the court she did not have proof they came from Mr Taylor and had given them to Mr Ractliffe because she wanted the stones to go to charity.
"Naomi suggested they could be of some benefit to the Nelson Mandela Children's Fund (NMCF) - but I told her I would not involve the NMCF in anything that could possibly be illegal," Mr Ractliffe said.
The fund has repeatedly denied receiving any diamonds from Ms Campbell.
Mr Ractliffe, who is still a trustee of the fund, said he took the diamonds as he thought it might be illegal for her to take them out of the country.
"In the end I decided I should just keep them," he said.
"A factor that influenced me not to report the matter to anyone was to protect the reputation of the NMCF, Mr Mandela himself and Naomi Campbell, none of whom were benefiting in any way."
Ms Campbell said at the trial in The Hague on Thursday that she spoke to Mr Ractliffe on the telephone in 2009, and he said he still had the stones.
Our reporter says it is possible that Mr Ractliffe and even Ms Campbell may have committed offences under South African law.
The legal code stipulates that any person without a permit who finds a rough diamond must forthwith hand it in at a police station.
Hawks' spokesman Musa Zondi told the BBC the gemstones had already been sent off for "verification and authentication".
If they proved to be "blood diamonds" a "serious investigation" would be set in motion and "everyone who was involved with the diamonds" would be questioned, he said.
Allegations that the uncut gemstones were given to Ms Campbell emerged in a statement by US actress Mia Farrow, who also attended the 1997 dinner.
Ms Farrow and Ms Campbell's former agent Carole White are due to appear before the Special Court for Sierra Leone at The Hague on Monday.
Mr Taylor is accused of using illegally mined diamonds to secure weapons for Sierra Leone's RUF rebels during the 1991-2001 civil war - a charge he denies.
Prosecutors say that from his seat of power in Liberia, Mr Taylor also trained and commanded the rebels.
The rebels were notoriously brutal, frequently hacking off the hands and legs of civilians.